[FEA] Improve testing of the AST engine across more data types and operators

[vyasr]

Is your feature request related to a problem? Please describe.
Currently the AST operators are generally well tested for correctness via the compute_column and mixed join APIs. All code paths are being hit for at least some data types. However, the matrix of data types and operators is vast and there are many code paths for specific data types that are not being tested.

Describe the solution you'd like
We should expand the set of types tested, probably via the transform tests since those are the easiest and cheapest. The main sets of tests that we should add are:

• Compare AST column-column binary ops to the vanilla cudf binary ops for all types that the latter supports.
• Compare AST column-scalar binary ops to the vanilla cudf binary ops for all types that the latter supports.
• Compare AST unary ops to the vanilla cudf unary ops for all types that the latter supports.